This invention relates to optoelectronic signal switching apparatus and in particular to a switching matrix which facilitates switching of multiplexed input signals to output lines.
The widespread introduction of optoelectronic signal switching matrices depends to a large extent on the cost of the crosspoint switch used in the matrix. It is highly desirable to use broadband switching crosspoints in the matrix, and particularly to use crosspoints with photodetectors which have gain. Where each broadband input signal must be switched at a single crosspoint to a single output line, clearly the achievement of these criteria, with a high quality photodetector, introduces high cost.
In the article by A. G. Foyt et al, "InP Optoelectronic Mixers, SPIE Volume 269 - Integrated Optics (1981), p. 189ff, (1981)" an InP optoelectronic switch is described in which the switch is used as a mixer. In this design a local oscillator signal is introduced by impinging on a photodetector, which signal modulated light, is mixed with an electrical input signal. This form of crosspoint switch however does not provide isolation between the output signal and the input signal as is highly desirable and is obtained in the crosspoint switch described in U.S. Pat. No. 4,286,171 issued Aug. 25th, 1981 invented by E. H. Hara et al, and in the crosspoint switch described in U.S. Pat. No. 4,369,371 issued Jan. 18th, 1983, invented by E. H. Hara et al.
In the article AVALANCHE OPTOELECTRONIC DOWN CONVERTER, by R. I. MacDonald and K. O. Hill, Optics Letters, volume 7, no. 2, February 1982, pages 83ff, the phenomenon of optoelectronic heterodyning is reported in an avalanche photodiode. A local oscillator signal is applied to a biased avalanche photodiode, which receives another signal simultaneously via a light beam (optical signal). We have found that this and other devices can be used to form an optoelectronic compound switch matrix as will be described below.
One of the ways of decreasing the cost of a switching crosspoint is to multiplex a group of input signals which are broadband switched at a switching crosspoint, and which are separated via demultiplexers connected to the output ports of the matrix. Such a switching matrix can be realized by the use of the crosspoint switch described in the aforenoted U.S. Pat. Nos. 4,286,171 and 4,369,371.
However in the present invention we have found a way to eliminate the costly demultiplexers and the wideband output ports. Instead, narrow band output ports can be used merely connected to lowpass or bandpass filters.